Download Chapter 1 The Framework of Biology

Chapter 6 The Engine of Life: Photosynthesis and Glucose
Metabolism
CHAPTER OUTLINE
6.1 Energy flows in metabolic pathways that link cells and organisms.
A metabolic pathway is a series of chemical reactions in which energy flows through
living organisms. Photosynthesis and glucose metabolism is one example of two
connected pathways through which energy is captured, stored and slowly released for use
in other metabolic reactions.
6.2 Photosynthesis harnesses the energy in sunlight.
Many types of organisms are capable of performing photosynthesis including some
bacteria, certain single-celled eukaryotes, algae and plants.
The biochemical pathways of photosynthesis take place largely inside chloroplasts.
Chloroplast structure provides areas and spaces in which the various parts of
photosynthesis occur. Thylakoid membranes provide areas for pigments and enzymes as
well as portioning spaces called the stroma. Certain wavelengths of light are captured by
the embedded pigments; chlorophyll is one of the most important.
The chemical reactions of photosynthesis are separated into two major processes: lightdriven reactions and carbon fixation reactions.
Captured light energy in the form of ATP and NADPH from the light driven reactions is
stored in part by the carbon fixation processes in the form of carbohydrate.
The light-driven reactions of photosynthesis use light energy to produce ATP and
NADPH.
Captured light energy in an electron is passed down one electron transport chain to form
ATP and another electron transport chain to form NADPH. Light driven reactions begin
with photosystem II in which pigments in an antenna complex gather light energy,
passing it to a reaction center of chlorophyll where an electron donated from the splitting
of water is sent to an electron transport chain. This electron transport chain produces
ATP using ATP synthase and ends in photosystem I. The splitting of water generates
oxygen gas which fills our atmosphere. Light energy is again absorbed in much the same
fashion as in photosystem II; the energy carrying electron is sent to an electron transport
chain which produces NADPH.
The carbon fixation reactions use ATP, carbon dioxide, and NADPH to make glucose.
The carbon fixation reactions occur in a cycle where the first molecule to which carbon
dioxide is fixed by the enzyme rubisco is regenerated. Energy is provided by ATP and
NADPH. The carbohydrate product is G3P, a molecule often converted to glucose.
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6.3 Cells extract energy from glucose.
Release of energy from glucose involves glycolysis, the generation of acetyl CoA, the
citric acid cycle and oxidative phosphorylation. Following the first step which occurs in
the cytosol, the mitochondria provides areas of membrane and spaces in which the
completion of glucose metabolism occurs.
Glycolysis breaks glucose into a pair of three-carbon molecules.
Glycolysis requires an initial input of energy as ATP but results in a net yield of energy
as ATPs as well as two three carbon pyruvate molecules.
The citric acid cycle releases the energy of acetyl CoA.
The 3 carbon pyruvate from glycolysis is converted to acetyl CoA which enters the citric
acid cycle. ATP is produced, NADH and FADH2 as high-energy electron carriers are
produced, carbon dioxide waste is produced and the initial molecule in the cycle is
regenerated.
ATP is made across the inner membrane of mitochondria.
Oxidative phosphorylation involves an electron transport chain embedded in a
mitochondrial membrane in which H+ ions are concentrated on one side of the membrane
using high-energy electrons from NADH and FADH2. ATP is formed by ATP synthase
powered by the movement of H+ ions from this highly concentrated area. Oxygen is the
required final electron acceptor and water is formed. This four-pathway process of
glucose breakdown is called cellular respiration.
Proteins and lipids can feed into the glucose metabolism pathway to provide cellular
energy.
Other organic molecules, i.e., lipids and proteins, can also enter cellular respiration and
provide energy.
6.4 Metabolic diversity reflects the evolution of energy pathways.
Many metabolic pathways are similar in structure and function among eukaryotes. The
example of the electron transport chain is discussed as a universal process found in living
organisms. Organisms which function in an oxygen environment have aerobic metabolic
pathways while those which function in the absence of oxygen have anaerobic metabolic
pathways. Clues from present-day organisms lead scientists in their discovery of how
metabolic processes evolved.
Metabolic diversity in eukaryotes gives you bread, alcohol and anaerobic exercise.
Many organisms function in anaerobic environments and undergo fermentation instead of
cellular respiration to produce ATP. Yeast can function in both aerobic and anaerobic
environments, producing carbon dioxide if oxygen is present. These processes are used
by humans to make bread in an oxygen environment or produce alcohol if oxygen is not
present.
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Muscle cells can use aerobic or anaerobic processes to produce ATP.
Muscle cells function mainly aerobically to produce ATP but switch to a fermentation
process resulting in less ATP and lactic acid if oxygen is unavailable.
Photosynthesis and cellular respiration produce and maintain Earth's atmosphere.
Both the production of oxygen from photosynthesis and carbon dioxide from cellular
respiration affect earth's atmosphere today and influenced early earth's atmosphere and
the evolution of living organisms.
6.5 How do you know? What is the connection between metabolism and global
climate change?
Human activity has an effect on global climate. Burning of fossil fuels, and other
activities which produce greenhouse gases, affect the atmosphere, in turn affecting the
climate.
Now you can understand.
Other effects of oxygen and the need for antioxidants are presented as well as the effect
cyanide has on oxidative phosphorylation.
What do you think?
A presentation of diet fads should stimulate critical thinking and discussion.
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